The present invention relates to an electron gun for cathode ray tubes, and, more particularly to an electron gun which enhances the resolution of cathode ray tubes by correcting astigmatism and focus characteristic variations due to the nonuniform magnetic field of a deflection yoke so that spots where the electron beam collides with a phosphor screen are uniformly formed throughout the screen.
As cathode ray tubes become large and flatter, the deflection angle of the electron beam emitted from an electron gun becomes larger. A larger deflection angle causes greater astigmatism due to a nonuniform magnetic field produced by a deflection yoke, and a larger halo due to differences in the focusing distance at the screen's perimeter with respect to its center. These effects gradually deteriorate the resolution of a cathode ray tube.
In order to solve these problems, a dynamic focusing method is provided for varying the focus voltage in synchronization with a synchronous deflection signal applied to the deflection yoke, which can be divided into two driving methods: a low voltage method and a high voltage.
FIGS. 1A and 1B illustrate an electron gun G using the low-voltage dynamic focusing method. The sequential composition of electron gun G is a cathode K, a control electrode G1 and a screen electrode G2 which together form a front triode, and a first focusing electrode G3, a second focusing electrode G4', and a third focusing electrode G5 which focus and accelerate the electron beams, plus an anode electrode G6. The second focusing electrode G4' is divided into a first auxiliary electrode G4a', a second auxiliary electrode G4b', and a third auxiliary electrode G4c'. In the first and third electrodes G4a' and G4c' are formed circular electron beam passing holes, and in second electrode G4b' are formed three horizontally elongated, rectangular electron beam passing holes Hv. A static screen voltage Ve is applied to screen electrode G2, first electrode G4a', and third electrode G4c'. A focusing voltage Vf which is higher than static screen voltage Ve is supplied to the first and third focusing electrodes G3 and G5. Static screen voltage Ve (lower than the static focusing voltage Vf) is supplied to the first and third auxiliary electrodes G4a' and G4c' of the second focusing electrode G4'. Parabolic dynamic focusing voltage Vd is applied to the second auxiliary electrode G4b', synchronized with vertical and horizontal synchronous signals of the deflection yoke, and takes the static screen voltage Ve as its base voltage.
According to the conventional low-voltage dynamic focusing electron gun G, when electron beams are not deflected toward the periphery of the screen of a cathode ray tube, that is, when they are projected towards the phosphor screen's center, the lowest dynamic focusing voltage Vd (Vd=Ve) is applied to second electrode G4b' of second focusing electrode G4'. Electron beams thus passing through second focusing electrode G4', maintain their cross section and form circular electron beam spots on the center of the screen.
When electron beams emitted from cathode K are deflected by the deflection yoke toward the periphery of the screen, dynamic focusing voltage Vd, varied according to horizontal and vertical synchronous signals and higher than the static focusing voltage Ve(Vd&gt;Ve), is applied to second electrode G4b. Thus, an intensive diverging lens is formed vertically by the horizontally elongated, rectangular electron beam passing holes Hv in the second focusing electrode G4' between the first and third focusing electrodes G3 and G5. The electron beams passing through the horizontally elongated electron beam passing holes have a vertically elongated cross section. When the electron beams are distorted by the deflection yoke's nonuniform magnetic field and land on the periphery of the screen, circular electron beam spots are formed.
Using dynamic focusing voltage Vd applied to second electrode Gb, the low-voltage dynamic focusing electron gun G compensates for astigmatism of beam spots which land on the screen when electron beams are deflected to the periphery of the screen. However, its compensating effect is too small to realize a sharp picture throughout the screen.